The present invention relates to a method for reconstructing a wideband speech signal from an input narrowband speech signal and, more particularly, to a method and an apparatus whereby a narrowband speech signal like present telephone speech or output signal from an AM radio can be graded up to a wideband speech signal like an output signal from an audio set or FM radio.
Telephone speech will be described as an example of the narrowband speech signal. The spectrum band of a signal that the existing telephone system can transmit is in the range of from about 300 Hz to 3.4 kHz. Conventional speech coding techniques are intended to keep the quality of speech in this telephone band and minimize the number of parameters that must be transmitted. Thus, it is possible with the conventional speech coding techniques to reconstruct band-limited input speech but impossible to obtain higher quality speech.
In Japanese Patent Application Laid-Open No. 254223/91 entitled "Analog Data Transmission System" there is proposed a system which transmits analog data after removing its high-frequency component at the transmitting side and reconstructs the high-frequency component at the receiving side through use of a neural network pre-trained in accordance with characteristics of the data. While this system transmits a narrowband signal of only the low-frequency band over the transmission line with a view to efficiently utilizing its transmission band, it can be said that at the receiving side the high-frequency component is reconstructed from the narrowband signal of the low-frequency component to recover the original wideband signal. The speech signal includes, however, spectrum information, pitch information and phase information, and it is unknown for which information the neural network has been trained; hence, there is no guarantee of correct reconstruction of the high-frequency component with respect to the data for which the network has not been trained. To train the neural network for all of such pieces of information, it is necessary to significantly increase the number of intermediate or hidden layers and the number of units of each layer--this makes it very difficult, in practice, to train the neural network.
With the recent progress of acoustics technology and development of digital processing, the quality of sound in everyday life has been improved and it has come to be said that the quality of speech in the telephone band at present is not satisfactory to many people. One possible solution to this problem is to replace the existing telephone system with a new one that permits the transmission of wideband signals, but this consumes considerable time as well as involves enormous construction costs.
It is therefore a primary object of the present invention to provide a wideband speech signal reconstruction method and apparatus which permit reconstruction of a wideband speech signal from an input narrowband speech signal transmitted with a view to efficient utilization of the existing telephone system, for instance, and which allow the use of a wideband speech signal even in a situation of the combined use of a wideband telephone system capable of transmitting a wideband signal and the existing narrowband telephone system.